Continuous mass production of various products, ranging from processed food to steel slabs, hereinafter called workpieces, has required development of quality control systems that maximize the detection of actual flaws in the workpieces while minimizing disruption to the production line that produces and uses the workpieces. As a consequence, sophisticated flaw detection systems have been developed which use, for example, TV cameras that view the workpieces as they travel along the production line, and which allow an operator to divert those workpieces that the operator concludes have flaws. An example of this type of inspection is illustrated in U.S. Pat. No. 4,209,802 which discloses a system for inspecting baby food jars for foreign matter using a TV camera to produce an image of each jar as it travels down a production line at the rate of one jar a second. This type of flaw detection system is adequate only to the extent that a human operator is alert to the flaw being inspected.
When the number of possible flaws per workpiece is large, as for example, when the workpiece is a printed circuit board, real time, on-line inspection of each workpiece becomes very difficult. As a result, inspection systems have been developed which use computer assisted processing of TV images of a workpiece for the purpose of detecting flaws. A typical system is disclosed in U.S. Pat. No. 4,570,180 wherein a camera scans printed circuit boards as they are produced on as production line, and digital imaging processing techniques are used to detect possible flaws.
Because the flaw detection procedures are less than 100% accurate, the conventional procedure is to consider automatically detected flaws as being only "possible" flaws subject to manual verification. This requires a dedicated verification station to follow an automated inspection station. Thus, in the conventional approach, the automated inspection system tags those boards that contain possible flaws and diverts them to the verification station where human operators visually verify the system's findings. Using such manual inspection, those workpieces containing false alarms that arise from surface contamination, for example, or false defects that arise from irregularities within specifications, are separated from the diverted boards, and returned to the production line.
The conventional approach described above has two main disadvantages: creation of discontinuities in the production line, and unnecessary handling of workpieces. To verify a possible flaw in a workpiece requires the latter to be withdrawn from the production line and transferred to a manual inspection station. Even if it develops that all of the flaws were false alarms, and there was actually no need to withdraw the workpiece from the line, time, effort, and unnecessary handling of the workpiece results. Any unnecessary handling of workpieces increases the potential for additional flaws that may or may not be detected at a later stage of manufacture; and this adversely affects the reliability and quality of the end product.
It is therefore an object of the present invention to provide a new and improved method of and means for inspecting workpieces which overcomes the problems set forth above.